Image-forming MR methods which utilize the interaction between magnetic fields and nuclear spins in order to form two-dimensional or three-dimensional images are widely used nowadays, notably in the field of medical diagnostics, because for the imaging of soft tissue they are superior to other imaging methods in many respects, do not require ionizing radiation and are usually not invasive.
According to the MR method in general, the body of the patient to be examined is arranged in a strong, uniform magnetic field whose direction at the same time defines an axis (normally the z-axis) of the co-ordinate system on which the measurement is based. The magnetic field produces different energy levels for the individual nuclear spins in dependence on the magnetic field strength which can be excited (spin resonance) by application of an electromagnetic alternating field (RF field) of defined frequency (so-called Larmor frequency, or MR frequency). From a macroscopic point of view, the distribution of the individual nuclear spins produces an overall magnetization which can be deflected out of the state of equilibrium by application of an electromagnetic pulse of appropriate frequency (RF pulse) while the magnetic field of the RF pulse extends perpendicular to the z-axis, so that the magnetization performs a precession about the z-axis. This motion of the magnetization describes a surface of a cone whose angle of aperture is referred to as flip angle. The magnitude of the flip angle is dependent on the strength and the duration of the applied electromagnetic pulse. In the case of a so-called 90° pulse, the spins are deflected from the z axis to the transverse plane (flip angle) 90°. The RF pulse is radiated toward the body of the patient via a RF coil arrangement of the MR device. The RF coil arrangement typically surrounds the examination volume in which the body of the patient is placed.
After termination of the RF pulse, the magnetization relaxes back to the original state of equilibrium, in which the magnetization in the z direction is built up again with a first time constant T1 (spin lattice or longitudinal relaxation time), and the magnetization in the direction perpendicular to the z direction relaxes with a second time constant T2 (spin-spin or transverse relaxation time). The variation of the magnetization can be detected by means of receiving RF coils which are arranged and oriented within the examination volume of the MR device in such a manner that the variation of the magnetization is measured in the direction perpendicular to the z-axis. The decay of the transverse magnetization is accompanied, after application of, for example, a 90° pulse, by a transition of the nuclear spins (induced by local magnetic field inhomogeneities) from an ordered state with the same phase to a state in which all phase angles are uniformly distributed (dephasing). The dephasing can be compensated by means of a refocusing pulse (for example a 180° pulse). This produces an echo signal (spin echo) in the receiving coils.
In order to realize spatial resolution in the body, linear magnetic field gradients extending along the three main axes are superposed on the uniform magnetic field, leading to a linear spatial dependency of the spin resonance frequency. The signal picked up in the receiving coils then contains components of different frequencies which can be associated with different locations in the body. The signal data obtained via the receiving coils corresponds to the spatial frequency domain and is called k-space data. The k-space data usually includes multiple lines acquired with different phase encoding. Each line is digitized by collecting a number of samples. A set of k-space data is converted to a MR image by means of Fourier transformation.
It is generally desirable to have a relatively uniform homogeneity of the generated RF field (B1 field) for excitation of magnetic resonance throughout a cross section of the imaged portion of the patient's body. However, as the MR frequency increases with increasing main magnetic field strength, this becomes more difficult due to conductive losses and wavelength effects within the body of the patient. Multi-channel transmit MR imaging has been accepted as a standard method of operating volume RF coils to achieve a relatively uniform B1 field. Compared with a single channel mode of operation, a two-channel transmit technique results in a significantly increased B1 homogeneity.
In known multi-channel transmit systems the RF power signal is typically supplied to the RF coil arrangement via RF drive ports being connected to individual resonator elements of the RF coil arrangement. The RF coil arrangement may be a so-called birdcage resonator comprising a plurality of rungs arranged in parallel to a longitudinal axis of the main magnetic field, wherein the birdcage resonator surrounds the imaged body portion. In this case, the RF drive ports are connected to two or more rungs of the birdcage resonator. Two-channel transmit MR systems typically use two independent RF transmit chains and amplifiers for applying the RF power signals to the RF drive ports of the coil arrangement. The RF power applied to the different RF drive ports can be controlled individually in order to optimize the homogeneity of the RF field (so-called RF shimming).
It turns out that different imaging tasks that utilize RF shimming have different RF power demands from each of the two channels. Dual channel shimming can therefore lead to a significantly asymmetric power demand at the two RF drive ports of the RF coil arrangement. The extent of asymmetry depends upon the anatomy in which the RF field homogeneity needs to be optimized and also upon the extent to which shimming is required. Typically, the RF power amplifiers within a multi-channel transmit system have equal power capabilities. However, the asymmetry occurring in the power demand results in a situation in which one of the RF power amplifiers does most of the work while the other RF power amplifiers idle.
Moreover, a drawback of known multi-channel transmit MR systems is that the asymmetry in the RF power demand changes according to, for example, a changed patient anatomy and/or a changed imaging task. For this reason it is necessary in conventional multi-channel systems to employ RF power amplifiers in all channels having sufficient excess power capabilities in order to be able to fulfill the requirements in any possible application. This results disadvantageously in increased system costs.